Clean up and modernize code:

This commit removes obsolete comments, dead or no longer useful
code, and workarounds for several issues that were present in older
compilers that we no longer support.

Specifically:

- It improves the transaction metadata handling class, simplifying
  its use and making it less error-prone.
- It reduces the footprint of the Serializer class by consolidating
  code and leveraging templates.
- It cleanups the ST* class hierarchy, removing dead code, improving
  and consolidating code to reduce complexity and code duplication.
- It shores up the handling of currency codes and the conversation
  between 160-bit currency codes and their string representation.
- It migrates beast::secure_erase to the ripple namespace and uses
  a call to OpenSSL_cleanse instead of the custom implementation.

src/ripple/protocol/STArray.h

@@ -30,14 +30,9 @@ class STArray final : public STBase, public CountedObject<STArray>
 private:
     using list_type = std::vector<STObject>;
 
-    enum { reserveSize = 8 };
-
     list_type v_;
 
 public:
-    // Read-only iteration
-    class Items;
-
     static char const*
     getCountedObjectName()
     {
@@ -48,7 +43,7 @@ public:
     using iterator = list_type::iterator;
     using const_iterator = list_type::const_iterator;
 
-    STArray();
+    STArray() = default;
     STArray(STArray&&);
     STArray(STArray const&) = default;
     STArray(SField const& f, int n);